High pressure abrasive water jet (AWJ) machining utilizes a very narrow stream of high pressure water laden with abrasive particles to erosion cut through a workpiece. AWJ machining is used in many industries, including the automobile, aerospace, computer, and glass industries, to create precision parts from a wide variety of materials such as plastics, metals, glass, composites, and ceramics, including those materials which are otherwise difficult to machine. The AWJ process machines with high precision, very little kerf, and produces a clean, smooth edge thereby reducing or eliminating the need for costly post-machining edge treatment operations. Because AWJ machining is a low temperature operation, it produces no heat affected zone in the machined part and can be used to machine heat treated parts without disturbing their heat treatment-induced material properties. AWJ machining heads may be guided by hand, machine, or computer with the most precise machining being obtained by computer-control of the AWJ machining head motion.
In a typical AWJ system, an intensifier pump is used to pressurize filtered water to the range of about 2,000 to 100,000 psi (14 to 690 MPa). The high pressure water is fed into an AWJ machining head where it is forced to pass through a nozzle orifice diameter as small as a few thousands of an inch (a few hundredths of a millimeter) to generate a high-velocity water jet. In commercial applications, abrasive particles such as garnet or olivine are introduced into the high-velocity water jet as it passes through a mixing chamber within the AWJ machining head. The abrasive particles and the high-velocity water jet mix as they travel together through the small diameter longitudinal bore of a mixing tube in the AWJ machining head to form upon exiting the mixing tube a narrow, abrasive, high-velocity water jet that is capable of making precise cuts through almost any kind of material.
An AWJ mixing tube longitudinal bore is subjected to severe jetting abrasion from the high-velocity water jet and abrasive particles it carries. However, the precision and the efficiency of AWJ machining is greatly affected by wear of the longitudinal bore of the mixing tube. Although the longitudinal bore diameters generally are on the order of 0.010 to 0.060 inches (0.25 to 1.5 mm) and the overall lengths of AWJ mixing tubes are usually on the order of 2 to 4 inches (5 to 10 cm), longitudinal bore diameter erosion of just a few thousands of an inch (a few hundredths of a millimeter) can greatly reduce the machining efficiency and degrade the machining precision, especially when the longitudinal bore erosion is near the exit end of the mixing tube. AWJ mixing tube longitudinal bore wear results in longer machining times, less precise machining, down time for replacing the worn mixing tube, and the cost of the replacement mixing tubes. To minimize this problem, AWJ mixing tubes are commonly made of a very hard materials, such as tungsten carbide.
In the past, there have been efforts to improve the wear resistance of AWJ mixing tubes by using chemically vapor-deposited (CVD) diamond as a longitudinal bore lining material. Diamond is an allotrope of carbon exhibiting a crystallographic network comprising covalently bonded, aliphatic sp3 hybridized carbon atoms arranged tetrahedrally with a uniform distance of 1.545 Å (0.1545 nm) between atoms and is extremely hard, having a Mohs hardness of 10. For example, Banholzer et al, U.S. Pat. No. 5,363,556, estimates that the use of diamond can extend the useful lifetime of AWJ mixing tubes from the about two to four hours obtained for conventional tungsten carbide mixing tubes to about twenty to one hundred hours.
Banholzer et al., supra, describes a method of making a AWJ mixing tube by depositing a diamond layer by CVD on a funnel shaped support member to form an inner member of diamond, separating the inner member from the support member, depositing an outer member material having a higher coefficient of thermal expansion than diamond on an outer side of the inner member to form an outer member of the mixing tube, and cooling the mixing tube to contract the outer member for inducing compressive stresses of sufficient strength on the inner member to substantially prevent the formation of cracks in the inner member. Anthony et al, U.S. Pat. No. 5,439,492, describes making a AWJ mixing tube by depositing a layer of diamond by CVD on a mandrel followed by removing the mandrel mechanically or by chemical etching to form the longitudinal bore of the mixing tube and then, optionally, providing a steel tube to support the diamond film. Stefanick et al., U.S. Pat. No. 5,785,582, describes depositing a layer of diamond by CVD on opposing sides of the longitudinal bore of a AWJ mixing tube made of a hard ceramic material that has been split longitudinally and then joining the two halves of the mixing tube together by shrink fitting a metal sheath around them.
There also have been efforts to use other forms of diamond and materials having hardnesses approximating that of diamond. Japanese Utility Model Application Laid-Open No. 63-50700, describes an AWJ mixing tube comprising a plurality of dies built in a sleeve main body. Each die consists of a knob of a polycrystalline sintered body of diamond or cubic crystal boron nitride, or the like, which is fixed to the inner circumference of an annular supporting stand metal of a tough material such as a super-hard alloy, high-speed steel, or the like. Each knob has a through-hole. However, the AWJ mixing tube described above has the disadvantage that wear occurs preferentially at the junction areas between the dies (see Examined Japanese Utility Model HEI-6-34936).